Radio communication, e.g., according to Long Term Evolution (LTE) specified by the Third Generation Partnership Project (3GPP) uses a Radio Link Control (RLC) Acknowledge Mode (AM) protocol to provide reliable delivery of Packet Data Units (PDUs) by means of an Automatic Repeat request (ARQ) procedure. A transmitting side transmits RLC PDUs with an RLC sequence number (SN) associated with each RLC PDU and polls the receiving side for a status report to acknowledge the RLC PDU was successfully received. An AM RLC entity at the transmitting side can receive an explicit negative acknowledgment (NACK) in a status report as a notification of reception failure by the peer AM RLC receiving entity for an Acknowledged Mode Data (AMD) PDU or for a portion of an AMD PDU, e.g., to trigger a retransmission of PDUs with RLC SNs for which the NACK was reported. There can also be implicit triggers for the retransmission, e.g., when the transmitting side times out waiting for the status report from the receiving side and is required to poll the receiving side but has no new data to send the poll request with.
When an AMD PDU or portion thereof is retransmitted, the RLC protocol of LTE requires the transmitting side to maintain one ReTx_count state variable in association with each SN. The ReTx_count state variable is incremented each time the RLC PDU with the corresponding RLC SN is retransmitted. If the ReTx_count for any RLC SN reaches maxReTx_Threshold number of retransmissions without receiving an acknowledgment (ACK) for the successful reception, a Radio Link Failure (RLF) declaration is indicated to the upper layers.
In New Radio (NR) according to 3GPP, the RLC layer works similar to LTE. One difference is that in NR the Medium Access Control (MAC) layer rather than the RLC layer concatenates upper layer data. More specifically, the MAC layer of NR multiplexes several Service Data Units (MAC SDUs) from the same logical channel. The AMD PDU from NR RLC thus contains either just one full RLC SDU (e.g., a Packet Data Convergence Protocol Data Unit or PDCP PDU) or just one segment of an RLC SDU (e.g., a segment of a PDCP PDU). Details of the ReTx_count for NR have not yet been decided in 3GPP.
One problem with existing RLC AM tracking of a ReTx_count per RLC SN is that it imposes a large memory burden in terms of amount of meta data and related processing cost just to provide a mechanism that detects there may be some radio link problem. Especially in NR, with concatenation removed from the RLC layer, and much higher data rates as compared to LTE, the RLC SN size will need to be increased, e.g., to 18 bits, compared to the RLC SN size of 10 bits in LTE. Maintaining a ReTx_count per transmitted RLC SN with such a large RLC SN space and at much higher SN rate adds a considerable burden on the memory requirements as well as increasing strain on the processing and number of memory accesses.
The purpose of the ReTx_count is to track the number of retransmissions of a Data Unit (DU) and compare it with a limit, maxReTx_Threshold, for continued retransmission. A notification, the RLF, is indicated to upper layers if the limit is exceeded.
However, this mechanism does not imply any control on the time that is required for the retransmissions. From an end-to-end service perspective, if the transmitting side has not received an ACK for all segments of an SN within a certain time period, the consequences are the same independent of details that cause the failure to deliver the DUs or to acknowledge their reception. Typically, the consequence to end-to-end service is based on time duration in which a higher layer packet or message cannot be delivered and is not dependent on low level details of the RLC protocol.
The actual time from the first MAC Hybrid ARQ (HARQ) failure until RLF is indicated may largely vary. The time for RLF indication depends not only on the settings of RLC parameters such as t-PollRetransmit and maxReTx_Threshold. Also other circumstances beyond the control of RLC may have an impact. The load in the system and other settings in a User Equipment (UE), such as Discontinuous Reception (DRX), influence when retransmissions can take place. Again, from an end-to-end service perspective, it does not matter what causes the transmission problem and it is just the time duration in which a higher layer packet or message cannot be delivered that matters.
The conventional mechanism for RLF detection based on a maximum number of retransmissions is also inefficient, because a lot of scheduling attempts are required to trigger the RLF. In LIE, a typical configuration for the UE in the uplink direction is to have a maxReTx_Threshold set to 32, meaning that it requires at least 32 scheduling attempts that each fail to be successfully delivered by the radio interface. Even worse, many more attempts may be needed in case different PDUs are scheduled for different re-transmission attempts. This means that, if the radio condition is such that RLF is not detected by other means, it will require quite a lot of wasted resources to trigger an RLF, because many scheduling requests must be granted for a UE, even though the radio access network knows that the radio condition relative to the UE is very bad. In LTE, the radio access network usually continues to schedule the UE even when it is in very bad radio condition so that the scheduling request is likely to fail, because it is important to trigger RLF for the maximum number of RLC retransmissions if required.